There is a related art apparatus for detecting flaws such as hairline cracks and defects or the kind of material of a test object using a sensor comprising a ferrite core on which primary and secondary coils are wound. This sensor is moved while being kept in contact with the surface of the conductive test object. A rectangular-wave current is supplied to the primary coil to induce eddy currents in a minute region on the surface of the inspected object. The eddy currents induce a magnetic field, which is detected by the secondary coil. The pulse duration of the voltage pulses detected by the secondary coil are measured. In this way, the flaws or the kind of material is detected. The principle of this detection utilizes the fact that the attenuation time of the eddy currents induced on the surface of the test object, i.e., the pulse duration, differs depending on the flaws such as hairline cracks or defects and on the kind of material.
The peak value and the pulse duration of the voltage pulse detected by the secondary coil are affected greatly by variations in the distance between the test object and the sensor. This makes it impossible to discern the sensor output signals indicating the flaws such as hairline cracks or defects and the material. Hence, the reliability of detection of flaws has presented problems.
Accordingly, it has been important for the flaw detection method and apparatus of this kind to reduce the effect of the aforementioned variations in the distance between the sensor and the object. For this purpose, various contrivances have been made. One example has been proposed by the present Applicant and is disclosed in Japanese Patent Laid-Open No. 316655/1989. In particular, the secondary coil output signal produces positive and negative driving currents in response to signal variations caused by variations in the distance between the sensor and the object. The driving currents activate driving coils disposed around a permanent magnet which is connected to one end of the sensor. In this manner, the distance between the sensor and the object is kept constant. However, the above-described driving currents must be supplied constantly. Also, large amounts of current must be supplied. Consequently, a large amount of electric power is consumed. In addition, the apparatus lacks portability. Furthermore, the responsiveness is a problem.